Overcurrent, undervoltage and overvoltage protection circuits have been separately used for power supplies for more than a decade. Each protection circuit has its own merits and limitations, such as function, cost and number of components. The advantages of the overcurrent protection circuits are that they are very simple, can completely shut-off the power supply (e.g., a power converter) and will remain off until the overcurrent condition is removed and input power is recycled. The function is desirable to protect the power supply from internal or external component failure and to prevent excessive heating in those failed components.
The undervoltage lockout function is either provided by a dedicated undervoltage circuit or by a pulse width modulation (PWM) integrated circuit (IC) controller. In either case, as the input voltage of the power converter increases from zero, the undervoltage lockout function will keep the power converter off until the input voltage rises to a predetermined level. Subsequently, as the input voltage drops from a normal operating input voltage to zero volts, the undervoltage lockout circuit will turn the power converter off when the input voltage falls below a predetermined threshold.
The undervoltage lockout function is desirable since the input current to the power converter tends to increase as the input voltage decreases. This function can effectively limit that current. Further, the undervoltage lockout function can prevent voltage drop and overcurrent shutdown in circuits with maximum duty cycle limiting. As the input voltage is lowered, the duty cycle increases, eventually reaching its maximum, causing the output voltage to fall out of regulation.
Unfortunately, some types of overcurrent protection circuits interpret this as an overcurrent condition and will latch the power supply off until the input power is recycled. As a result, the power supply could latch off or have low output voltage during various input voltage conditions, such as slowly rising input voltage or momentary input voltage loss.
An example of circuits that provide overvoltage protection is overvoltage clamping circuits. Overvoltage clamping circuits function in a number of different fashions. In one instance, a sensor detects a higher than expected voltage at the output of a power converter and a portion of the overvoltage clamp circuit clamps the output at a maximum voltage. Once clamped, the overvoltage clamp circuit continues to hold the output voltage at the clamped value, only allowing the voltage to drop, not rise any higher. Additionally, some circuits also have overvoltage shutdown abilities. If so, another portion of the overvoltage circuit eventually forces the power converter to shut down if the overvoltage condition persists thereby protecting the power converter and/or its load from damage due to high voltage levels.
An example of a power converter requiring the aforementioned protection functions is hereinafter described. For some power supply applications, a controller that limits a duty cycle of the switching devices of the power converter to fifty percent is desirable. The switching devices such as the metal-oxide semiconductor field-effect transistors (MOSFETs) and Schottky diodes of a resonant reset forward converter, for instance, may suffer very high voltage stresses during the start-up process and other transient conditions. The high voltage stresses are a result of the large duty cycle (e.g., eighty-five percent) imposed on the switching devices of the power converter. Consequently, very little time in one cycle remains to reset the transformer and, therefore, the voltages of the transformer, MOSFET and diodes have to be extremely high to reset the transformer. The high voltage stresses often contribute to the failure of the MOSFETs and diodes. To eliminate this problem, a controller (e.g., a PWM IC) that limits the duty cycle of the switches to about fifty percent is employed for a forward converter with resonant reset. Further, when using peak current control converters, a lower maximum duty cycle is desired since these converters are known to have inherent instabilities where duty cycles of greater than fifty percent are employed.
Accordingly, what is needed in the art is a recognition that merging protection functions for a power converter is advantageous and, what is further needed in the art, is a circuit that combines at least overcurrent and undervoltage protections in an integrated fashion.